Answer:
Length: 150 yards
Width: 50 yards
Explanation:
The perimeter is equal to the twice the length plus twice the width. If the perimeter is 400, then the length+width is equal to 200. Then if length is 3 times the width, then l = 3w. The width is just w so l+w=4w and 200/4w is 50, so the width of the field is 50 yards. The length is 3 times the width, so the length is 50*3=150.